The field of small space positioning, positioning within spaces of a few meters or less, is known. Known applications utilizing small space positioning systems include applications with pointing devices for computer interaction, robotics and machine control systems, as well as computer interaction systems for toys and inventory control. Certain applications may require 2D solutions, others may require 3D solutions. Certain applications such as pointing devices may require only one-way communication, whereas others, e.g. robotic applications, may require two-way communication.
Digital writing instruments, e.g. digital pens, are typically used to capture and digitize pen strokes for computer interaction. Known digital pens include ultrasonic based digital pens. Typically, an ultrasonic pulse signal put out by the digital pen is sensed by receivers and distances determined from the outputs of the receivers are triangulated and correlated to absolute pen positions. Typically, the position measurements are based on measuring the Time Of Flight (TOF) of the transmitted pulses through a Line Of Sight (LOS). TOF estimation typically requires accurate synchronization between the transmitter and the receiver to compensate for clock inaccuracy and/or drift.
International Patent Application Publication No. WO2005111653, entitled “Acoustic Robust Synchronization Signaling For Acoustic Positioning System”, assigned to Epos Technologies Ltd; Altman Nathan, the contents of which are hereby incorporated by reference, describes positional element and positioning device, wherein the positional element transmits a continuously modulated acoustic waveform and a synchronization signal that is a sequence of at least two synchronization packets, each bearing timing data for the continuously modulated acoustic waveform. Additionally, the synchronization signal uses time hopping to support concurrent positioning of a plurality of positional elements.
International Patent Application Publication No. WO03088136, entitled “Method And System For Obtaining Positioning Data”, assigned to Epos Technologies Ltd; Altman Nathan and Eliashiv Oded, the contents of which are hereby incorporated by reference, describes a positional element for attaining a position and including a first emitter for emitting a substantially continuous ultrasonic waveform decodable to fix the position, and a detector arrangement for detecting the waveform in a manner permitting fixing of the position and outputting the waveform for computation, in a manner retentive of the position fixing ability.